1. Field of the Invention
This invention relates to crossover interconnects used in high voltage integrated circuits and structures that prevent junction breakdown caused by high voltage differences between the crossovers and underlying semiconductor junctions.
2. Description of Related Art
High voltage integrated circuits are useful in many applications including power ICs. In a typical high voltage IC, the voltage difference between two junctions formed in the same semiconductor can be high. For example, in a power IC which supplies power to a household appliance, voltages differences of 110 volts AC (or about 500 volts absolute) are common. In other applications, voltage differences are even higher. High voltage differences cause strong electric fields in the semiconductor and can cause junction breakdown. Consequently, two circuit elements in a high voltage IC that have a high voltage difference must be separated or shielded from each other.
FIG. 1 shows a typical pair of doped regions 101 and 102 formed in a semiconductor substrate 100 as part of a high voltage IC. When the region 101 is at a low voltage level, zero volts, and the region 102 is at a high voltage, such as 500 volts, there is a large voltage drop between the junctions 121 and 122. The voltage drop causes an electric field which forms in a region 103 between the junctions. The electric field and the change in voltage are related by the equation .DELTA.V=.intg. E.multidot.dx. With a fixed voltage, increasing the distance between the junctions decreases the magnitude of the electric field. If the electric field near the junction 121 or 122 is weak enough a depletion region around the junctions 121 and 122 prevents unwanted current from flowing. If the electric field gets too strong near a junction 121 or 122, junction breakdown occurs and unwanted current flows.
The problem of junction breakdown becomes worse when high voltage crossovers are used. In FIG. 1, the doped regions 101 and 102 underlie a crossover 104 which is separated from the regions 101 and 102 by an insulating layer 106. Crossovers are generally made of metal or some other conductor and therefore have substantially uniform voltage along their lengths. However, the voltage level on the crossover may vary with time. For example, the voltage on the crossover 104 may change with time back and forth between 0 and 500 volts.
If, for example, the region 101 is at 0 volts, the region 102 is at 500 volts, and the crossover is at 0 volts, the voltage drop along a path 105 through the insulating layer 106 is 500 volts=.DELTA.V=.intg. E.multidot.dx. Because a large voltage drop takes place over a short distance, the electric field along the path 105 and at the junction 122 is strong, and a junction breakdown may occur. The breakdown problem is exacerbated by an image charge formed near the surface of the semiconductor caused by the crossover 104. The image charge aids current flow between the regions 101 and 102 if junction breakdown occurs. If the voltage on the crossover goes to 500 V, the electric field near the junction 122 is weak, but a strong electric field near the junction 121 may cause a breakdown similar to that discussed above.
The problem of junction breakdown caused by high voltage crossovers is critical in high voltage ICs, because use of crossover interconnects are an efficient way to connect elements of an IC. Many solutions have been tried to address the junction breakdown problem.
One solution uses individual wire bondings to make connections between elements. Connection wires which connect from bond to bond can be kept at a greater distance from the junctions. However, individual wire bondings for every structure that requires an interconnect makes manufacturing complicated and expensive.
Another solution is routing the high voltage interconnects to avoid junctions that may be affected. This solution wastes silicon "real estate" because junctions must often be widely separated to provide paths for the interconnects. The paths are often complicated and convoluted.
Conducting field plates have also been used. Field plates may be floating or set to the voltage of a junction. The field plates are placed between the junction and the crossover. A strong electric field develops between the field plate and the crossover, but if the field plate is properly designed, the junction is shielded from the strongest electric fields and breakdown is prevented. The primary disadvantage of field plates is the cost of additional steps required to manufacture a high voltage IC and that the available oxide thicknesses in the process may be incompatible with those thicknesses desired for high breakdown voltage devices.
Semi-Insulating Poly Oxide Silicon (SIPOS) layers have also been used to prevent breakdown. SIPOS layers are placed in contact with the region surrounding the junction. When a crossover generates an image charge adjacent to a junction, the SIPOS drains excess charge away and prevents a strong electric field from forming in the area of the junction. The disadvantages of SIPOS are the extra manufacturing steps required to fabricate the high voltage IC and the power lost during operation from the current flowing in the SIPOS layer.